The invention is of primary advantage when applied to the field of ink jet recording of the type similar to that shown in Sweet et al U.S. Pat. No. 3,373,437 where pressurized electrically conductive fluid is ejected from one or more rows of orifices for breaking into streams of uniform drops. As each drop breaks off from its fluid filament as ejected from the corresponding orifice, it may be selectively charged by an associated charge electrode. The charge electrodes are thus arranged in rows corresponding to and spaced from the rows of orifices so that the fluid streams may pass therethrough. The drops then pass through an electrostatic deflection field and each drop is deflected a distance which is related to the magnitude of the drop charge. As an example, the drops may be charged binarily so that the uncharged drops are undeflected and continue past the deflection field to impact a recording medium for printing while the charged drops are deflected to a drop catcher or gutter.
The deflection field is formed by at least two spaced apart deflection electrodes of different voltages. In some instances, such as the above U.S. Pat. No. 3,373,437, a grounded deflection plate is porous to also serve as the drop catcher to catch deflected ink drops impacting thereon. The high voltage electrode is solid, however, and is designed to repel the charged drops. A control electrode in other types of ink jet printing such as Hertz U.S. Pat. No. 3,673,601 is also porous to catch charged drops which are diverted from the normal path of uncharged drops. The field is established between the ink and the control electrode and no sets of electrodes are provided to form a deflection field. Only one ink jet is employed and misting is not a substantial problem. A two row ink jet system is shown in Mathis U.S. Pat. No. 3,701,998 employing a high voltage center deflection electrode and two separate grounded electrodes for establishing the deflection fields and also serving to intercept the deflected ink drops. All of the deflection electrodes are solid and not porous, however.
When a large number of closely spaced ink jets are employed for recording, a significant amount of mist is generated in the vicinity of the recording medium. A substantial portion of the mist tends to collect on the deflection plates with the result that ink accumulates rapidly on a center deflection plate between two rows of ink jets. It is desirable to minimize the distance between the two adjacent rows of ink jets so as to simplify the data manipulation requirement for compensating for the time required for the relative movement of one row of jets to the position on the recording medium previously encountered by the other rows of jets. Hence, it has been extremely difficult to clean the center deflection plate.